README updates (#856)
README updates (#856)

* README: Remove mentions of master branch

* README: Add instructions for using html-test

* README: Change command to run _all_ the testsuites

* README: Add project overview section

(cherry picked from commit 61d6f935da97eb96685f07bf385102c2dbc2a33c)

haskell/haddock,haskell/haddock,haskell/haddock,haskell/haddock,haskell/haddock